The present invention is directed to a magnetic slip clutch system that can be used in a number of different applications where a slip clutch is desired to be used; and particularly, to applications where the clutch is desired to be fully disengaged, i.e., free-wheeling, fully engaged to transmit 100% of the power, or coupled in a modulated or slip mariner for transmission of a variable amount of power. The torque transfer is modulated by controlling the electric power to the clutch. One use of such a clutch is disclosed in copending, Provisional patent application Serial No. 60/095,750, filed Aug. 7, 1998, entitled "Magnetic Clutch Method and Apparatus for Driving a Vehicle Air Conditioner". The present invention will be described in connection with another application of the magnetic slip clutch as a fan clutch, which could be used with fans used to cool electric motors as well as internal combustion engines.
In a typical large truck in the United States, having a liquid cooled internal combustion engine, the fan clutch is either disengaged completely with two clutch faces separated or fully engaged with the two clutch faces transmitting power across the clutch face interface without any slippage between the faces. Usually, the clutch faces are engaged or disengaged pneumatic or electromagnet actuators. When the clutch is engaged in the large truck, considerable power is used to drive the fan. Where the fan is a 32" diameter fan, the power used to drive the fan can be as much as 40-80 horsepower for large trucks which have engines in the range of 200-600 horsepower. On the other hand, when the fan is fully off, the fan is disconnected from the power drive from the engine's crankshaft. The fan is free-wheels and does not contribute to cooling of the engine. In these particular type of on/off electromagnetic and air-actuated fan clutches, the fan is disconnected when the engine is cold or is being adequately cooled, for instance when air is forced rapidly through the radiator by high speed travel of the vehicle.
The control system for the fan clutch includes a thermostat, which monitors the engine or the cooling fluid temperature and tries to maintain the temperature within a predetermined range-- for example, 181.degree.-190.degree. F.); and the control system includes a speed measuring sensor which disengages the clutch when the vehicle speed reaches a set speed--for example, 30 mph. At 30 mph, it is assumed that there is sufficient ram air flow across the radiator to cool the engine without operation of the fan.
With large trucks which are running at 2,100 rpm and which use 40-80 horsepower to drive a large fan having a large movement of inertia, e.g., a 32-inch diameter fan, it is necessary to build a fairly substantial fan clutch to withstand the shock of suddenly engaging and rotating the inertia of a stationary, driven clutch face and its attached fan quickly to its full rotational speed. Consequently, current fan clutches are quite large and substantially made to withstand such shock loads.
The typical ON/OFF conventional fan clutch used or large trucks or the like employs metal machine parts which are relatively heavy and take considerable horsepower to accelerate and to overcome the inertia thereof. For example, when the fan is of a 32" diameter, the horsepower used to drive the fan can be as much as 40 to 80 hp for large trucks which have engines in the range of 200 to 600 hp. The machining of the fan clutch metal pieces also adds considerably to the cost of this fan clutch. The ON/OFF fan clutches also must be relatively strong and made of metal to withstand the torque impulse forces which are applied relatively quickly, e.g., in about 1.2 seconds, when the fan clutch goes from fully off, light torque load to a very high, full torque load to accelerate the fan-carrying part to its maximum speed, which is generally about the engine speed which can be 2,400 rpm. Thus, it is necessary to build a fairly substantial metal fan clutch to withstand the shock of the sudden inertia load of the stationary driven clutch base and its attached fan. Consequently, current ON/OFF fan clutches are quite large and are substantially made to withstand such shock loads and high forces when the fan blades are being accelerated to rotate at high speeds and subjected to large centrifugal force loads at high rotational speeds. The metal pulley and fan carrying pieces require considerable machining to provide the desired surfaces, reference locations, and bearing seats. Such machining adds considerably to the cost of this clutch.
The ON/OFF fan clutches use frictional faces which wear and generally need to be replaced after 150,000 miles or so in order to meet the warranty requirements, which warrant the fan clutch for about 250,000 miles. Of course, the replacement of the friction faces results in a substantial amount of down time during their removal and their replacement. In contrast thereto, the electromagnetic fan clutch uses only rotational parts rather than frictional parts and uses only a magnetic field to transfer the torque of the rotating input pulley part to the driven rotational part carrying the fan blades. With current ON/OFF fan clutches, the sixty or so horsepower used by the onset of the fan load and the considerable horsepower used to accelerate the fan under an impulse load, for example, of 60 hp to turn a large 32" fan, is noticeable for large trucks climbing a grade. When the fan clutch comes on and takes 60 hp, the engine must slow down and often the driver must shift down two gears to keep the vehicle going. There is a need for a relatively soft engagement which can be controlled so as to avoid the full application of 60 hp over the short impulse time of 1.2 seconds or the like, thereby eliminating the need to downshift.
It appears that these current ON/OFF fan clutches may be engaged at least 12% of the time for large trucks or the like. Since the large truck is using the full extent of the fan horsepower with the ON/OFF clutch, there may be as many as 40 to 80 hp being used to drive the fan when there is relatively small need for full engagement, because a partial engagement would satisfy the cooling needs. There is a need for an electromagnetic fan clutch that can supply a modulated torque of less than full torque and more than a minimal torque and would be engaged for a lesser time period, e.g., only about 1% of the time. Further, the magnetic clutch should be able to be modulated to increase the torque transfer gradually to eliminate the large shock loads of the ON/OFF fan clutch.
An additional consideration with the ON/OFF fan clutch is that of the large amount of noise generated by the fan which is rotating at high speeds for 12% of the time. It is estimated that as much as 50% of the noise from a large truck emanates from the fan which is beating or slapping the air as it rotates at about engine speed to cool the engine. A reduction in noise level using a modulated fan clutch could open new markets for equipment using fans and fan clutches where the equipment cannot be used in the evenings or at night if it exceeds regulated noise levels.
In addition to such load problems from the large movement of inertia, another problem with current fan clutch systems is that of chattering, i.e., an abrupt, recurring, on/off engagement of the clutch fan faces due to a rapid opening and closing of a thermostat switch trying to maintain the cooling temperature between 181.degree. F. and 190.degree. F. Typically, the on/off thermostat has a variation of seven degrees (7.degree. F.) with a tolerance of plus or minus two (2.degree. F.) degrees between turning on or shutting off. In some adverse, ambient conditions of operation, this cycling goes back and forth as many as 50 times per hour, thereby causing wear and tear on clutch faces, when they are being repeatedly mechanically engaged and disengaged.
When the clutch is operated by an air pressure system from the truck's compressed air system, other problems arise. That type of clutch uses compressed air to operate the clutch actuator system. It is difficult to keep the compressed air clean. The truck compressor air is usually too dirty to be used directly to operate the fan clutch actuator system. The air must first be cleaned by bubbling it through oil in an oil filter which traps dirt from the air. If the oil in the filter becomes dirty, it will pass contaminants. The still-contaminated air may compromise a downstream pressure regulator resulting in an insufficient air pressure for operating the clutch actuator, e.g., the air pressure may drop below 70 pounds and not provide sufficient force to operate the clutch. If the fan is not operated, the temperature can reach 450.degree. F. at the fan blades. The plastic fan may ignite setting the entire truck afire. pneumatic fan clutch systems are bulky and expensive.
With the current fan clutch systems in large trucks in the United States, the driver turns on the air conditioning system for the truck and the fan clutch is automatically engaged to turn the fan at full speed even though full fan speed may not be needed for adequate cooling of the engine. For example, the ram air flowing through the radiator at high vehicle speeds could be sufficient to keep the air conditioner and the truck engine temperature low enough without the fan. While engaging the fan clutch when the air conditioner switches on ensures that the air conditioning system and the engine are kept cool at all times, it wastes energy.
Current fan clutch systems in trucks have the fan mounted fairly high which keeps the nose or the front end of the vehicle high. Truck manufacturers and truck drivers want to lower the truck nose for streamlining the aerodynamics of the truck and to provide a better view of the road. In some trucks, the radiator has been turned on its side so that it is a side-flow radiator in order to lower the tractor nose profile. In some tractors, there is insufficient space to position a large fan on its side and place it between the frame rails for the vehicle. Hence, the fan cannot be side-mounted to drop it lower.
These electromagnetic and pneumatically-clutched fans are not directly connected to the engine crankshaft because the crankshaft vibrates at a natural frequency that causes the engaged faces of the clutch plates to vibrate against one another and to wear quickly. Thus, most trucks in the United States have a fan belt drive from the crankshaft to the fan clutch which is located above the crankshaft. A fan belt drive, however, takes space, and the belt is subject to failure. The fan belt system also adds cost to the engine cooling system. Although many truck manufacturers would prefer to lower the radiator and fan to the location of the crankshaft, they are unable to do so because of the crankshaft vibrations and the space needed for the fan belt drive of the fan clutch and fan.
In Europe and in other places such as South America there is often used a viscous fluid fan clutch which is always rotating at speeds of 400 to 600 RPM, even when the fan is turned off. That is, the viscous fluid rotates the fan because of the friction and shears. When the viscous fan clutch is turned on, it never is able to produce or transfer 100% of the input power or torque because 7-10% of the power is lost with viscous fluid shearing. Because of the fluid viscosity and friction, this fan clutch is never totally disengaged to be free-wheeling as it is constantly engaged so that it is always using a considerable amount of the power. This power, of course, is wasted fuel consumption, which makes it a relatively inefficient fan clutch from a fuel economy standpoint. This particular fan clutch also uses a bi-metallic thermostat on the front of the fan assembly to measure temperature and a plunger is operated by the thermostatic switch, which requires ram air to operate. On slow moving construction vehicles or the like where there is relatively little ram air, such a thermostatically controlled fan clutch is not readily usable. Likewise, for an ON/OFF fan clutch used on trucks there is a requirement for the use of compressed air which is often not available for construction or farm equipment and therefore makes the clutch less saleable to makers of such equipment.
Currently, it is desired to eliminate the 7-10% viscous shear inefficiency without an increase size of the radiators and to provide this increased efficiency to power the fan in order to run-the engines hotter using the same cooling equipment. The magnetic fan disclosed in the aforesaid patent application can be used with these hotter engines because it transfers 100% of the power using a magnetic field and does not have the 7-10% inefficiencies due to the viscosity of the viscous fluid.
In all of these fans and fan clutch systems, removal of heat is a significant problem for the bearings and for the fan belts. The failure of bearings or the breaking of fan belts are a considerable cause of maintenance or down time. By keeping the bearings and fan belt temperature lower, the life of the belts and bearings can be improved considerably. These are important considerations with respect to the maintenance and the overall down time with respect thereto caused by bearing or belt failures. The replacement of a bearing on a fan clutch is a significant maintenance problem for a truck or a piece of equipment. Thus, there is a need for a new and improved magnetic clutch that overcomes the deficiencies of existing ON/OFF frictional face clutches and existing viscous fluid clutches.
Some vehicles, principally in Europe, have a fan clutch which is mounted on the crankshaft and which is always partially engaged to transfer a certain amount of power to the fan. For example, at least 40% of the power to turn the fan to full speed needed and up to 90% of the power to turn the fan at full speed. This clutch never transfers 100% of the power, and this clutch is never totally disengaged such that the fan can be free-wheeling. This particular type of clutch also has a vibration isolator which to some extent serves to dampen or isolate vibrations at the fan clutch from the crankshaft vibrations. If the vibration isolator wears out allowing vibration to be transmitted, the clutch will wear out in only a few days.
Such a constantly engaged fan wastes fuel. If the fan consumes a maximum of 40-80 horsepower, and the fan never uses less than 40% of its input power, this means that at least 16 horsepower is being consumed that may not be needed. At the higher end, 40% of the 80 horsepower is 32 horsepower which may not be needed but it is always being expended. This unneeded expenditure of energy results in fuel inefficiencies for vehicles having such fan clutches.
Similarly, in most automobiles, the fan runs continually while the vehicle engine is running. Such continuous fan operation wastes fuel, particularly when the engine is moving at a sufficient speed to provide ram air which would cool the engine; or when the engine is cold and there is no need for any further cooling by a fan.
It will be appreciated that pneumatic or electromagnetic actuators for bringing the clutch faces into engagement are relatively complex, occupy considerable space, heavy and costly. The controls using the thermostat on and off switch as well as a speedometer sensor on/off switch provide the most rudimentary information because more information would not enable any more sophisticated thermal management due to the fact that the clutch cannot be modulated in its torque transfer and thereby achieve proportional control of engine temperature. Also, it will be appreciated that the amount of horsepower being used for a fan cooling system is very substantial in that great fuel economy savings could be accomplished by having an improved fan clutch and control system for the internal combustion engine cooling.